For the processing of tree trunks, a harvester head, i.e. a delimbing and cutting apparatus, is used for the purpose of gripping an upright growing tree, cutting the tree and felling it, after which the tree trunk is delimbed and cut into pieces of fixed length by means of a sawing device. A harvester head with a feed roller is disclosed in WO publication 00/15025. The harvester head is normally connected to the end of the crane of a forest working machine. The harvester head is connected to the crane by means of a joint, and it comprises the necessary actuators, normally hydraulic cylinders and hydraulic motors, for controlling the position of the head and its different functions. The harvester head comprises delimbing members which can be articulated in relation to the frame structure and which comprise delimbing blades for delimbing branches while the trunk is supported and forced through the apparatus. The feeding members comprise a feed roll or a feed track assembly which is pressed against the trunk and pulls it through the apparatus. The harvester head also comprises cutting members, for example a chain saw, for cutting the tree trunk.
Delimbing and cutting apparatuses are also known which are suitable for the handling of tree trunks already felled, for delimbing the tree trunk and cutting it into pieces of fixed length.
A rubber feed roller is disclosed in document WO 95/01856. A feeding device comprising a crawler is disclosed in document U.S. Pat. No. 3,669,161. In WO 99/41972 and Fl 97340 B, there are four feed rollers, wherein the feed roller motors of the same side are coupled in series and the feed roller motors of opposite sides are coupled in parallel. A mechanical parallel coupling is used to prevent divergence of the rotational speeds of the motors and, among other things, skid.
The feed motors have normally a fixed displacement, wherein the feeding speed is constant and only depends on the volume flow supplied to the motor. The relatively small size and light weight of the feed motors are suitable in view of the harvester head, aiming at lightness and ease of steering. However, the limited feeding speeds constitute a disadvantage.
In Fl 101868 B, in turn, dual-capacity motors are applied as feed motors for a delimbing and cutting apparatus, wherein the apparatus comprises two feeding members, for example feed rollers. A dual-capacity motor is disclosed in U.S. Pat. No. 6,099,273. Said motor is applied primarily in the power transmission of vehicles.
The dual-capacity motor is a radial piston motor comprising an input and output connection as well as an auxiliary connection which can be used as an input or output connection. The motor also comprises a selector, i.e. a stem in a drillhole, by means of which some of the pistons direct the used volume flow to the normal output connection and the other pistons feed it to the auxiliary output connection. In this way, the motor has at least two different capacities (dual-capacity motor), wherein it comprises, in a way, two half-motors. Alternatively, the auxiliary connection can be an auxiliary inlet connection, through which the volume flow is supplied to one of the half-motors. Because of the common shaft, however, the rotation speeds of the half-motors are the same. Said selector can also be missing, in which case the motor always has three connections available, one being connected to all the pistons and the two others being connected to specific separate pistons only, wherein the speeds to be achieved will depend on the couplings with which the motor is controlled.
The coupling of dual-capacity motors, for example in the coupling according to document Fl 97340 B, is problematic, because two motors coupled in series are used, and furthermore, one of them is coupled mechanically to a third motor. Speed differences between the feed rollers must be avoided particularly in the use of dual-capacity motors coupled to the system to provide different feeding speeds.
For example, when dual-capacity motors are used, one of the half-motors must normally be let on free circulation to provide different speeds. However, in the design of the free circulation, one must take into account that the half-motor on free circulation will affect the efficiency and the energy economy of the whole system. The effect is the greatest when the half-motor on free circulation causes a significant loss of pressure.
Another factor to be considered in the operation of feed motors coupled in series is cavitation which is caused when the pressure of the liquid used as the pressurized medium drops to a sufficient degree, for example as a result of flow resistance in the suction channel. The cavitation will cause quick wearing of the equipment because of cavitation erosion.